At present, with the popularization of network, digital contents such as electronic books, electro-newspaper, electric journals and digital video and audio have been accepted by more and more users, and the digital library is widely concerned in colleges, communities and even public networks. Thus, the copyright protection of digital content is becoming more and more important for publication houses, newspaper offices and libraries. The current problem is how to realize the copyright protection of digital content by time control in applications such as security protection of digital libraries and electronic documents. For instance, when a user “borrows” a digital content from the digital library, the digital library only allows the user to access the digital content within a particular time period. Therefore, the digital rights management system should correctly determine the “current time” when a user accesses the digital content: the user can access the digital content if the current time is within the designated time range; otherwise, the digital content is deemed as being expired, and the user cannot access said digital content.
The key to the realization of the digital content time control depends on the acquisition of a correct “current time” and avoidance of malicious amendments to the “current time”. In order to realize the above purpose, the following solutions are usually adopted:
(1) The first solution realizes time control by combining the system time with a certain algorithm. Since the system time can be modified, and BIOS of the computer device can be cleared, it is unreasonable to determine whether the digital content are expired or not by means of the system time.
(2) The second solution realizes time control by modifying the certificate through writing the lapsed time into it. The disadvantage of this solution is as follows: if the user accesses the initial certificate to overwrite the modified certificate, the expired digital content can be used again.
(3) The third solution is recording the time difference when the user modifies the time. A DRM system performs correction according to the time difference (e.g. see the method of the patent application titled “Digital Rights Management Method And System For Content Copyright Protection” applied by Samsung Electronics Co., Ltd. in February, 2005). The disadvantage of this solution is that it is closely combined with the system hardware, for instance, it is hard to be realized a universal PC platform. Additionally, if the time of the device is not accurate, and the user performs correction according to a standard time, the system will consider it as a malicious time modification and record the time difference.
The above time control solutions all make it possible that the expired user employs a certain method (and it can be easily done) to access the expired digital content. If the expired digital content can still be used, the copyright protection of the digital content by means of time control cannot be realized.
Additionally, there are other solutions which record time by designing a special hardware device for achieving the purpose of time control. However, the method of designing a special hardware device is not suitable for a universal device, and it will increase the burden of common users and go against the popularization of digital contents. Therefore, universal devices such as PC require safe, convenient and practical time control methods.